Engine Rattle, higher RPM's!

[KewlBird]

Well to start off it's a 1990 Mercruiser 5.0LX 230hp....with alpha 1.
The engine runs well and idles fine.....After I have it at 3/4 to full throttle for a little bit of time (3-5 minutes) the engine starts to rattle...If I move the throttle back to about 1/2 the rattling goes away! This is when engine is warm or cold. A few days ago I was on it and it started rattling, then puffed a couple of times (as if backfiring) then stalled out! It restarted fine....the temperature is normal when it does it and the oil pressure is ok. I'm guessing that it could be a lean mixture, but wouldn't it rattle all the time when you're on it and not after 3-5 minutes? It feels like it has less power when it does this......BTW, I replaced the fuel filter and it didn't change anything...Could it be the fuel pump not supplying enough fuel? Thanks.

 

[jtybt]

Re: Engine Rattle, higher RPM's!

Sounds like detonation. DO NOT RUN IT UNTIL YOU FIGURE OUT THE PROBLEM!!!!!!!!

 

[dedhead63]

Re: Engine Rattle, higher RPM's!

Could be bad gas or fouled plugs..

 

[wca_tim]

Re: Engine Rattle, higher RPM's!

could be several things, most much better to figure out and correct than to wait until stuff flys apart...

 

[KewlBird]

Re: Engine Rattle, higher RPM's!

I keep adding new gas to the tank without emptying it completely so gas could have some old gas still in there (it's a 90 gallon tank)......The spark plugs are brand new (it did it with old ones too), cap and rotor are new as well....
Detonation is caused by too advanced timing right? I adjusted the base timing to where it's supposed to be...(it's at either 6 or 8 degrees....)
Argh! The only thing that I'm thinking of is fuel pump isn't keeping up...

 

[jtybt]

Re: Engine Rattle, higher RPM's!

If it is detonation, it can be caused by too far advance, too high compression for fuel grade or lean condition.

Check timing with good advance timing light with tach at the RPM that you're having trouble.

Lean condition can be bad fuel or wrong jets, plugged transfer tubes in carb., wrong float setting, intake air leak, fuel line air leak, fuel line restriction and more.

 

[Bondo]

Re: Engine Rattle, higher RPM's!

Lean condition can be bad fuel or wrong jets, plugged transfer tubes in carb., wrong float setting, intake air leak, fuel line air leak, fuel line restriction and more.

Ayuh,... Start by looking for Water in the fuel Filters...

 

[BAproject]

Re: Engine Rattle, higher RPM's!

If you replaced the distributor cap did you verify that the timing was still good? I would get a timing light very soon and sync up the timing!

-BAproject

 

[KewlBird]

Re: Engine Rattle, higher RPM's!

Ayuh,... Start by looking for Water in the fuel Filters...

When I replaced it couple of weeks ago the fuel in the filter was somewhat yellowish, not much just not as clear as regular fuel.....Would this indicate water? If not what's the best way to determine whether the stuff in the fuel filter is fuel or water?

If you replaced the distributor cap did you verify that the timing was still good? I would get a timing light very soon and sync up the timing!

-BAproject

I verified the timing and it was dead on......

 

[KewlBird]

Re: Engine Rattle, higher RPM's!

Any chance you are running a low or multi-viscosity oil?

I'm running SAE 30....

 

[KewlBird]

Re: Engine Rattle, higher RPM's!

Well I'm going to recheck timing one more time, empty fuel filter and if I can get to the fuel pick-up line I'll check if there is any dirt in there....

 

[rbh]

Re: Engine Rattle, higher RPM's!

It sure is nice to use a timing light that shows you when total advance comes in, because you never know if the advance is working properly or not.
rob

 

[KewlBird]

Re: Engine Rattle, higher RPM's!

It sure is nice to use a timing light that shows you when total advance comes in, because you never know if the advance is working properly or not.
rob

Well, my timing light will show the advance timing up to 60 degress......
I just rechecked and timing at idle ~700 rpms was at a little over 10 degrees....I'm not sure why it increased since last time! Everything is tight and both times I checked the timing the engine was warm. I adjusted it back down and now I'm sitting at about 7 degrees. It should be at 8, but I figured if it was because of the timing this will help eliminate the problem..... I also checked the advance at 3600-3700 which is where about the problem kicks in and the advance was right at 30 degrees. The fuel pick-up tube is not removable but I did peek inside the tank through the sending unit hole....The fuel was very light yellow in color and I didn't really see any sediments at the bottom of the tank!....Didn't have enough time to dump the fuel out of fuel filter.... got to work.

 

[KewlBird]

Re: Engine Rattle, higher RPM's!

Good grief, what sort of boater allows work to get in the way of boating?!?!?!

Haha! I have to be able to refuel the boat and the truck to get the boat into the water somehow...

 

[meesh]

Re: Engine Rattle, higher RPM's!

Dump your fuel filter into a glass jar and let it sit overnite. If there is water there, they will seperate.

 

[Apollo75]

Re: Engine Rattle, higher RPM's!

Well, if I could hear a rattle running high RPM over the other noise in the boat -- I would slow down.

Could be just about anything !

I run PREMIUM fuel --- always have --- always will --- premium fuel is processed further and burns even --- regular gas has hydrocarbons in it that burn faster --- so you get a knock, ping, rattle, as these hydrocarbons burn fast or detonate (explode) / or / pre-ignition from the high heat in the combustion chamber before the plug actually fires. I would start here first --- better fuel -- can't hurt a thing.

A marine engine is running under load at all times --- at 4500-rpm much more load and much more combustion heat than 3000-rpm. A car can be doing 5000-rpm and only be making a little horsepower --- part throttle while shifting through the gears --- a boat at 5000-rpm is turning a prop and is under considerable load.

OFM

 

[Apollo75]

Re: Engine Rattle, higher RPM's!

Step back non-believers

http://en.wikipedia.org/wiki/Octane_rating

"Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner."

"It might seem odd that fuels with higher octane ratings are used in more powerful engines, since such fuels explode less easily. However, an explosion is not desired in an internal combustion engine. An explosion will cause the pressure in the cylinder to rise far beyond the cylinder's design limits, before the force of the expanding gases can be absorbed by the piston traveling downward. This actually reduces power output, because much of the energy of combustion is absorbed as strain and heat in parts of the engine,[citation needed] rather than being converted to torque at the crankshaft."

Enough said:

OFM

 

[KewlBird]

Re: Engine Rattle, higher RPM's!

Quick update: I took the boat out today, before removing the fuel filter....and after it hit 3,400rpms it started to rattle, pretty much immediately this time.....That kind of eliminates the timing issue. I just dumped the fuel filter into a jar and it looked like there were some sediments that were in the filter (I barely have any time on this filter), I still have to wait and see if there will be any water separation.... I will get a new filter in the mean time.

 

[jtybt]

Re: Engine Rattle, higher RPM's!

Have you done a compression check?

 

[bruceb58]

Re: Engine Rattle, higher RPM's!

I run PREMIUM fuel --- always have --- always will --- premium fuel is processed further and burns even

This is actually an incorrect statement. It isn't processed further. Its the same exact fuel and stays the exact same fuel until products that increase the octane are added to it. The surce of my info is my long time friend who is a chemical engineer from Chevron who only uses regular fuel.

The only engines that need premium fuel are those with higher compression. If you engine doesn't ping, it doesn't need premium.

I agree with Wikipedia's description. Did you know that fuels have lower octane at higher elevations? You actually want a lower octane number then. Your engine will actually have more power with the lower octane.